Mineral Sulphides of Iron. 215 



would bring a two-layered system of this character to homo- 

 geneity, but at present the problem is experimentally beyond 

 our means. 



Summary. 

 [Including the results of Part III, as follows.] 



1. The formation of iron disulphide was accomplished (1) by 

 the action of hydrogen sulphide on ferric salts, or the action 

 of sulphur and hydrogen sulphide on ferrous salts ; (2) by the 

 addition of sulphur from solution to amorphous ferrous sul- 

 phide or pyrrhotite ; (3) by the action of soluble polysulphides 

 on ferrous salts ; (4) by the action of soluble thiosulphates 

 on ferrous salts according to the equation 4M 2 S 2 3 + FeX„= 

 3M a S0 4 + FeS a +2MX + 3S. The first three methods may be 

 generalized as the action of sulphur on ferrous sulphide : (a) 

 in acid solutions; (h) in nearly neutral solutions, and (c) in 

 alkaline solutions, since in (1) we may assume that ferrous sul- 

 phide first forms by the action of hydrogen sulphide on the 

 ferrous salt, and in (3) we know that polysulphides first pre- 

 cipitate a mixture of ferrous sulphide and sulphur. Marcasite 

 was obtained with certainty only by method (1) ; low tempera- 

 tures and free acid favor its formation. A solution containing 

 about 1 per cent of free sulphuric acid at 100° gives pure mar- 

 casite. Less acid solutions at higher temperatures give mix- 

 tures of marcasite and pyrite. The other methods give pyrite 

 which, under certain conditions, may be mixed with amorphous 

 disulphide. It is possible that some marcasite may be formed 

 by method (4). 



2. Marcasite and pyrite were identified in the above products : 

 (1) By microscopic examination and crystallographic measure- 

 ment. The pyrite crystals showed only the cube and the octa- 

 hedron. Marcasite crystals were formed for the first time. 

 They were commonly twinned after (110), and their habits are 

 shown in figures 15 and 16, Pt. III. The axial ratios of the syn- 

 thetic mineral are a:b:c = 0*7646 : 1 : 1*2176 and agree remark- 

 ably well with those of the natural mineral. (2) By Stokes's 

 oxidation method, which serves also for the analysis of mix- 

 tures of the two minerals. 



3. Marcasite changes to pyrite with evolution of heat. The 

 change proceeds very slowly at 450° and is not accelerated by 

 pressures even of 10,000 atmospheres. Marcasite is mono- 

 tropic toward pyrite. This is in accord with the greater incli- 

 nation of marcasite to oxidize, its assumed greater solubility, 

 and the fact that its formation is conditioned by the composi- 

 tion of the solution from which it crystallizes. 



4. The fact that marcasite never occurs as a primary con- 

 stituent of magmas, while pyrite sometimes does, is explained 



